Conventionally, there are various types of glow plugs. Examples of such glow plugs are a metal glow plug in which a heater is configured by housing a heating coil in a tip end portion of a bottomed cylindrical metal pipe, and a ceramic glow plug using a ceramic heater in which an insulative ceramic is used as a substrate of the heater, and a heating element made of a conductive ceramic is embedded in the substrate. All of such glow plugs are used in, for example, preheating of a diesel engine. Conventionally, a glow plug is used with being attached to an engine in a manner where a heating portion at the tip end of a heater is projected into a sub-combustion chamber.
Recently, a demand for a ceramic glow plug among such glow plugs is particularly growing in accordance with a request for high-temperature resistance because of enhancement of the engine performance.
As shown in FIG. 7, for example, such a ceramic glow plug comprises: a ceramic heater 130; a metal shell 110 having a thread portion for attachment to an engine head; an outer cylinder 160 which holds the pressingly inserted ceramic heater 130; a pin terminal 170 through which an electric power is supplied from the outside to the ceramic heater 130; a center pole 120; a lead coil RC; an insulating member 140 which ensures insulation between the center pole 120 and the metal shell 110; and an O-ring 150 which is pressed against the insulating member 140 to maintain an airtight seal of the interior of the metal shell 110 (see JP-A-2003-56848).
In the thus configured glow plug, a cord for supplying a power from a battery which is not shown is connected to the pin terminal 170 in the rear end of the glow plug. The power is conducted through the center pole 120, the ceramic heater 130, the outer cylinder 160, the metal shell 110, and the engine head or the ground. In the glow plug having this configuration, even when the combustion pressure from the combustion chamber which is due to combustion in the engine acts on the ceramic heater 130 toward the rear end in the axial direction, the lead coil RC can relax a stress of the ceramic heater. Therefore, the ceramic heater 130 and the center pole 120 can be prevented from being destroyed or damaged.
In another configuration, in place of the lead coil RC, a cylindrical member 121 such as shown in (b) of FIG. 7 is used, and the center pole 120 and the ceramic heater 130 are mechanically rigidly connected to each other in a direct manner. It is described that, according to the configuration, even when the thickness of the cylindrical member 121 is decreased in order to reduce the diameter of the glow plug (particularly, in the case where the diameter of the thread portion of the metal shell is not larger than M8), a predetermined sectional area can be obtained, and therefore insulation can be maintained while ensuring the gap between the heater 130 and the metal shell 110, without forming the heater into a step-like shape (see JP-A-2003-130349).